Telephone substation apparatus



Jan. 23, 1940; R. H. HERRICK ET AL 01 TELEPHONE SUBSTATIdN APPARATU-S miwmr mm mm 1 n 5555" am w P E I L m, l

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Patented Jan. 23, 1940 PATENT OFFICE TELEPHONE sUBs'rA'rIoN APPARATUS Rtswell Harry Benick and Turner Wright Gilman, Oak Park, 111.,

Electric Laboratories, poration of Delaware assignors to Associated Inc., Chicago, 111., a cor- Applicatlon April 18, 1938, Serial No. 202,662

27 Claims.

The present invention relates to telephone 'substation apparatus and has for one of its objects Y the provision of an improved arrangement for incorporating an'auxiliary loud-speaking sound reproducing unit in a telephone substation circuit.

A substation oi the character mentioned conventionally comprises a telephone transmitter and receiver, a hooker cradle switch, a ringing device and an auxiliary loud-speaking sound reproducing unit including amplifying means and arranged for alternative use with the telephone receiver. When the substation is to be used in a telephone system of the automatic type, there 'is also included in the circuit thereof a dial operated impulsing device for transmitting switch control impulses over the associated line to the automatic switching equipment provided in'the line terminating exchange. In the improved substations of this type, anti-side-tone circuit or impedance networks are provided for preventing operation of the transmitter from. effecting operation of the receiver when the receiver is connected for use and for preventing operation of the transmitter from effecting operation of the auxiliary sound reproducing unit when this unit is connected for use. Various substation arrangements of this character have, in the past, been used; and while, for the most part, these prior arrangements are reasonably satisfactory in operation, they are, in the usual instance, rather complicated in character and expensive.

It is another object of the present invention to provide an improved telephone substation circuit which is equipped alternatively to provide amplified or non-amplified reproduction of audio frequency currents transmitted to the substation, and which is of simple and economical arrange-, ment and requires a minimum amount of circuit equipment.

It is a further object of the invention to provide in a substation circuit of the character mentioned a change-over relay operative to alter the substation circuit arrangement so that amplified or non-amplified reproduction of received audio frequency currents may selectively be obtained and which is also operative to'cause a biasing voltage to be impressed between the input electrodes of one or more of the amplifier vacuum tubes included in the substation circuit.

It is a still further and more specific object of the invention to provide a telephone substa tion circuit includingv an anti-side-tone impedance network and apparatus for optionally providing amplified or non-amplified audio frequency output currents from the substation circuit,

which is so arranged that current for energizing the transmitteris fed to the substation transmitter over the line extending to the substation and audio frequency energy developed by operation of the transmitter is not delivered to the 5 line through the current feed circuit.

In .the illustrated embodiment of the invention the substation circuit is shown as comprising a transmitter, a receiver, and an anti-sidetone impedance network arranged to be inter- 10 connected to form a substation circuit. The anti slde-tone impedance network includes a plurality of inductively coupled impedance elements connected to form a four-arm bridge circuit and is operative, when the receiver is connected in the substation circuit, to'prevent audio frequency energy developed by operation of the transmitter from being delivered to the receiver. Also included in the substation is an auxiliary sound reproducing unit and achange-over relay which is operative to substitute the auxiliary sound reproducing unit for the receiver in the substation circuit. A second amplifier is provided which, in combination with the anti-side-tone impedance network, functions to deli er audio frequency energy developed through operation of the transmitter to the line extending to the line terminating exchange. In accordance with one feature of the present invention, the changeover relay, which operates to substitute the aux- 3o iliary sound reproducing unit for the receiver in the substation circuit, is also operativeto impress a biasing voltage between the input electrodes of at least one vacuum tube included in each of the two amplifiers. More particularly, as power supply apparatus including a power rectifier is provided for delivering energy to each of the two amplifiers, which apparatus includes an input circuit arranged to be completed in response to the operation of a manually operable 4 switch. The change-over relay includes a winding connected and arranged to be energized in -response to the completion of the power apparatus input circuit, and circuits are provided, individual to the amplifier tubes and commonly including the relay winding, for impressing a biasing voltage between the input electrodes of each of the amplifier tubes. In accordance with another feature of,the present invention, a circuit including a portion of the impedance ele- 5e ments embodied in the anti-side-tone impedance networks and the conductors of the line incoming to the substation circuit, is provided for conducting energizing current by way of the line to the substation transmitter. This circuit includes 5s a network for eifectively short-circuiting the current feed circuit for audio frequency currents developed by operation of the transmitter, thus preventing audio frequency energy from being delivered to the line by way of the current feed circuit and effectively providing a closed, low impedance, voice frequency transmission circuit.

Further features of the invention pertain to the particular arrangement of the circuit elements whereby the above and additional operating features of the circuit are attained.

The novel features believed to be characteristic of the invention are set forth with par.- ticularity in the appended claims. The invention. both as to its organization and method of operation, together with further objects and advantages thereof, will best be understood by reference to the specification taken in connection with the accompanying drawing in which there is illustrated in partially schematic form an automatic telephone system having incorporated therein a substation connected and arranged in accordance with the present invention.

Referring now to the single figure of the drawing, there is diagrammatically illustrated a telephone system comprising an automatic telephone exchange 10 which may, if desired, constitute av private automatic exchange. Terminating at the exchange [0 are a plurality of telephone lines, two of which are indicated at H and I2. The ends of the lines remote from the exchange i8 terminate in substations of the automatic type. that associated with the line H being diagrammatically illustrated at A, and that associated with the line 12 being indicated generally at B. The automatic exchange It] comprises the usual subscriber controlled automatic switches for setting up connections between the telephone lines of the system. More particularly, these switches are, in accordance with conventional practice, divided into two classes, namely, nonnumerical switches such, for example, as line finder or line switches, and numerical switches, such, for example, as selector and connector switches, which are responsive to impulses transmitted thereto over calling ones of the telephone lines and function to perform the usual line group, subgroup and unit selecting operations. The connector switches at which the lines terminate are, in accordance with conventional practice, equipped to perform the usual auxiliary functions of testing a selected called line to de- 'termine the idle or busy condition thereof, returning busy tone over the calling line in the event a selected called line is found busy, and projecting ringing current over a selected called line to actuate the signal device at the called substation in the event a selected called line is idle at the time it is tested.

Referring now more particularly to the equipment provided at the substation B, this equipment comprises a ringing or vsignal device 13 connected across the line conductors l6 and I! of the line i? in series with a condenser l 5, a transmitt-er 18. a receiver IS, an impulsing device 20. and switching means in the form of a hook or cradle switch 2!. The transmitter and receiver preferably are mounted as a unit to form a hand set of the usual type. The switch 2! comprises a member 22 for supporting the receiver 19 when not in use and switch springs 23 and 24 of which the springs 23 are operative in response to the removal of the receiver i 9 from the supporting member 22. to place a direct current bridge across the conductors of the line l2, thereby to create a calling condition on this line, and operatively to include the receiver IS in the substation circuit. The impulsing device 2(iis of the conventional dial operated type and comprises a pair 01' impulsing springs 25 and a set of shunt springs 26, which latter springs are normally restrained to the open circuit position indicated in the drawing and are so arranged that the dial mechanism closes the same during operation of the impulsing device. Also included in the substation circuit is an auxiliary sound reproducing unit comprising a translating device or loud speaker 21 coupled to the output electrodes of a vacuum tube amplifier including a vacuum tube 28 of the well-known pentode type. More particularly, the input terminals of the loud speaker 21 are connected to the secondary winding of a coupling transformer 29 having a primary winding connected between the anode 30 and the oathode 3! of the tube 28 in series with a source of anode potential comprising the power supply apparatus described more particularly hereinafter. The vacuum tube 28 also includes input electrodes comprising a cathode 3| and a contrp l grid which are coupled to the adjustable portion of a voltage dividing resistor 32.

Incoming signal currents are delivered to the receiver I 9 or to the auxiliary sound reproducing unit through an anti-side-tone impedance network indicated generally at 33. This network comprises a plurality of impedance elements 34, 35, 36, 31 and 38 which are inductively coupled to each other in the manner illustrated. The first four of the enumerated elements are interconnected to form a bridge circuit having conjugate terminals between which is connected the secondary winding 39 of a coupling transformer 40, the arrangement being such that currents traversing the secondary winding 39 divide and traverse the two arms 34 and of the bridge in opposite directions. The winding 38, which is arranged to be coupled to the receiver l9 or to the input terminals of the auxiliary sound reproducing unit, is so coupled to the two windings 34 and 35 that, when altermati g currents of equal magnitude and opposite phase traverse these two windings, substantially no voltage is developed between the terminals of the winding 38. For the purpose of balancing the equivalent impedances, measured at the terminals of the substation B, of the impedance elements external to this substation when the substation is involved in an established communication connection, there is provided a balancing network comprising a condenser and an adjustable resistor 42. More particularly, these two elements are suitably proportioned to balance the equivalent impedance of: the shunt and series impedances of the line [2, the impedances of the coupling elements included in an established switch train, the shunt and series impedances of a second line connected More particularly, the vacuum tube 41 is provided with output electrodes comprising an anode 48 and a cathode 49 which are arranged to be connected directly across a series circuit including the transformer, winding 43'and the source of anode potential comprising the power supply apparatus described hereinafter. The tube 4'! also includes input electrodes comprising the cathode 49 and a control grid 50. These electrodes are coupled to the adjustable portion of a voltage dividing resistor 5| shunting the secondary winding 46 of the coupling transformer 44.

The above-described outgoing signal current amplifier is provided in order to permit the transmitter l8 to be used as the microphone irrespective of which of the translating devices I 9 and 21 is used as the receiving means. In this regard and as pointed out in detail hereinafter, when the auxiliary sound reproducing unit including the loud speaker 27 is operatively included in the substation circuit, the hand set comprising the transmitter I8 and the receiver I9 is not removed from the supporting cradle 22. This means that a person must speak into the transmitter from a distance when the receiver is not being used. Hence, the outgoing signal amplifier is required to step up the level of the audio energy delivered to the line 12 during operation of the transmitter and when the hand set including the transmitter is not removed from its cradle.

The arrangement is such that current for energizing the transmitter I8 is conducted from the exchange battery provided in the exchange ID by way of the windings of the line relayembodied in an operated one of the numerical switches also included in the exchange Hi, the line conductors l6 and I! of the line l2 and the two inductance elements 34 and 31 of-the network 33 to the transmitter [8. In order to prevent audio frequency energy developed by operation of the transmitter ill from being delivered over the circuit just described to the line H, there is provided an audio frequency current blocking and by-pass network comprising a choke coil 52 and two by-pass condensers 53 and 54 individually having capacitance values such that the impedance thereof to audio frequency currents is exceedingly low.

The power supply apparatus, mentioned above,,

includes a voltage step-down transformer 55 having a primary winding 56 included in the input circuit for the apparatus, which circuit also comprises a source of alternating current, not shown, but arranged to be connected between the input terminals 51. The transformer 55 also includes a low voltage secondary winding 58 for supplying energizing current to the cathode heaters of the two vacuum tubes 28 and 41, a low voltage secondary winding 59 for energizing a signal lamp 60 when the transformer primary winding 56 is energized, and a low voltage winding 6! for energizing the cathode heater 62' of a full wave rectifying tube 62. The vacuum tubes 28, 41 and 62 are preferably of the type embodying directly heated cathodes for the reason that a tube of this type inherently requires less time for its cathode to reach its normal electron emitting temperature following the energization, of the cathode heater. The tube 62 is in the manner illustrated to form a conventional filter circuit.

Inworder to substitute the auxiliary sound reproducing unit for the receiver"! in the substation circuit and for the additional purpose of operatively including thev signal amplifier comprising the vacuum tube 41 in the path for delivering audio frequency energy from the transmitter It! to the line I2, there is provided a relay 12 which includes a' winding arranged to be energized in, response to the completion of the input circuit to the power supply apparatus.v More particularly, this relay includes a winding connected in series with the resistor 13 between the negative power supply conductor 66 and the center tap of the low voltage secondary winding 58. The series connected resistor I3 and winding of the relay 12 are by-passed for audio frequency currents by a condenser 14. With the winding of the relay 12 connected in this manner, it will be noted that this relay is operativeto impress a biasing voltage between the input elec trodes of each of the two tubes 28 and 41. More particularly, the voltage drop developed across the resistor 13 and the winding of the relay 12 when this winding is energized is impressed between the cathode and control grids of each of the two tubes noted to drive the cathodes of the respective tubes positive with respect to their respective associated control grids. The resistor 73 is provided for the purpose of permitting a relay of standard construction and having a fixed resistance to be used, while providing the requisite biasing voltage for the input electrodes of the two tubes 28 and 41.

The relay I2 is provided with a pair of armatures I5 and 16, the first of which cooperates with an associated contact 11 and is operative from resting engagement with this contact to disconnect the receiver I9 from the substation circuit and operatively to include the auxiliary sound reproducing unit in this circuit. The second armature 15 is operative between a resting contact 18 and a working contact 19 to include the second amplifier comprising the vacuum tube 41 in the path for transmitting audio frequency energy from the transmitter I 8 to the line It. More particularly, the armature l5 normally engages the resting contact '18 so that the amplifier tube 41 is excluded. from the transmission path noted, the transmitter I8 normally being coupled to a closed link circuit comprising the secondary winding 46 of the coupling transformer 44, a condenser 80, and the secondary winding 43 of the transformer 4|]. When, however, the relay 12 is energized, this link circuit is interrupted and the output electrodes of the tube 41 are connected across the winding 43 of the transformer 40, it being observed that the input electrodes of this tube are at all times coupled to the adjustable portion of the voltage dividing resistor 5|.

For the purpose of preventing the substation circuit from singing, due to electrical and acoustical coupling between the energy receiving and energy transmitting channels, there is provided a relatively large condenser 8| which is arranged effectively 'to short-circuit the transmitter l8, during intervals when the network 33 is unbalanced. In order to complete the input circuit to the power supply apparatus and to place a bridging path across the conductors I6 and I1 of the line I2 independently of the hook or cradle switch 2|, there is provided a key operated calling switch 82 which includes two sets of switch springs 83 and 84. There is also provided a key operated answer switch 85 including a pair of springs 88 which are operative to open the low impedance circuit including the condenser 8| normally shunting the transmitter I8, thereby to condition the substation circuit for operation. The two key operated switches 82 and 85 are of the locking type, and means comprising a suitable mechanical interlock, indicated by the dotted line 81, are provided, whereby the switch 82 may be operated to the off-normal position without affecting the position of the switch 85, the switch 82 is operated to its off-normal position in response to operation of the switch 85 to the off-normal position, and both switches are simultaneously restored to normal in response to the operation of the switch 85 to its normal position. More particularly, the switch 82 is operative to its off-normal position to close the two sets of springs 83 and 84 without operating the switch 85 to its off-normal position. Operation of the switch 85 to its oiT-normal position, however, causes the switch 82 to be operated to its off-normal position. With both of the switches operated to the off-normal position, and when the switch 85 is returned to normal, the mechanical interlock functions to cause the switch 82 also to be restored to its normal position.

Referring now to the operation of the system and assuming, for example, that a call is initiated at the substation A and routed by way of the automatic switching equipment provided in the exchange I and the line l2 to the substation B at a time when the last-mentioned substation is idle, when the line I2 is seized by one of the connector switches provided at the exchange In and tests idle, ringing current is projected over the called line to actuate the ringing device I3, thereby to give an audible indication that the substation B is being called. The party answering the call has the option of employing the receiver I9 or the auxiliary sound reproducing unit to transform incoming audio frequency currents into sound energy. If the party answering the call elects to employ the receiver l8 as the sound reproducing means, this receiver is removed from its supporting member 22 in the usual manner. The hook switch 2 responds to this operation by closing its contact springs 23 and opening its contact springs 24. At the springs 24, the low impedance shunt comprising the condenser 8| normally connected across the transmitter I8 is interrupted. At the springs 23, a direct current bridging path is completed between the conductors of the line I2, this path extending from the line conductor I 6 by way of the inductance element 34, the transmitter I8, the transformer winding, 45, the choke coil 52, the transformer winding 39, the inductance element 31, the impulsing springs 25 and the closed springs 23 to the opposite line conductor IT. The completion of this bridging path results in the operation of the connector switch to arrest the ringing operation and to complete a talking connection between the calling and called substations in the usual manner.

When the talking connection is completed, the exchange battery provided in the exchange I0 is connected between the line conductors I6 and I1 in series with the windings of the back bridge relay conventionally provided in the operated connector switch, so that current for energizing the transmitter I8 is delivered by way of the line conductors I6 and I1 and through the two inductance elements 34 and 31 to the transmitter circuit .in the manner pointed out above. This transmitter circuit is effectively closed and comprises the primary winding 45 of the coupling transformer 44 and the low impedance by-pass condenser 53. Due to the low impedance of this condenser, the low impedance of the condenser 54, and the high impedance of the choke coil 52 to audio frequency currents, substantially no audio frequency energy is fed back over the transmitter energizing circuit and through the two inductance elements 34 and 31 of the network 33 to the line 2. The purpose of preventing such feed-back of energy and of providing the low impedance path comprising the condenser 53 is to increase the transmitting efficiency of the transmitter I8. More particularly, if all of the impedances included in the circuit over which direct energizing circuit is supplied to the transmitter I8 were connected in series with this transmitter, very little audio voltage would be developed across the primary winding 45 of the transformer 44, and the transmitting efficiency of the substation circuit would be very poor. By virtue of the provision of the audio frequency bypassing and blocking network comprising the three elements 52, 53 and 54 substantially the full audio frequency voltage developed by operation of the transmitter I8 is impressed across the terminals of the transformer primary winding 45.

With the key operated answer switch 85 in its normal position, audio frequency currents traversing the transformer winding 45 produce corresponding induced voltages in the secondary winding 48 which cause corresponding currents to flow in the closed link circuit comprising the winding 48, the condenser 80, the resting contact 18 and armature I of the relay I2, the primary winding 43 of the coupling transformer 40 and the condenser 1|. These currents cause corresponding induced voltages to be developed in the secondary winding 39 which are impressed on the conjugate terminals of the anti-side-tone bridge through the condenser 54. The currents produced as an incident of these induced voltages divide between the inductance elements 34 and 35, substantially one-half of the current traversing a path extending by way of the inductance element 35, the condenser 4|, the resistor 42, and the inductance element 36, and the other half of the current traversing the inductance element 34. the loop extending through the automatic exchange III to the substation A, and the inductance element 31. Through the provision of the balancing network comprising the condenser 4| and the resistor 42, the currents respectively traversing the inductance elements 34 and 35 are rendered substantially equal in magnitude and opposite in phase at all frequencies within the operating band. Since the inductance element 38 is substantially equally energized by the currents of equal magnitude traversing the two elements 34 and 35, substantially no audio frequency voltage is developed across the terminals of the element 38 during operation of the transmitter. Thus, the well-known anti-side-tone effect is realized.

The path traversed by voice frequency currents incoming to the substation B extends from the line conductor I6 by way of the two inductance elements 34 and 35, the condenser 4!, the regy is delivered through the coupling transformer sistor 42, the two inductance elements 36 and 31, the impulsing springs25 and the switch springs 23 to the opposite line conductor I1. Qbviously', C

portions of the incoming currents are by-passed around the two elements 35 and 36 through the branch circuit comprising the condenser 4| and the secondary winding 38. Inthis case thecurrents traversing the two elements 34 and 35 are in aiding phase relation so that substantial corresponding voltages are developed across the terminals of the element 38 which are impressed across the receiver 19 over a circuit including the armature 16 andtheassociated resting con-' tact 11 of the relay 12.

In the event the answering party elects to employ the auxiliary sound reproducing unit as the receiving means, the receiver. I9 is retained on its supporting member 22 and the key operated answer switch 85 is operated to its ofi-normal position to open the springs 86, thereby to interrupt the low impedance path comprising the condenser 8| normally shunting the transmitter l8. Due to the above-described mechanical in-i terlock between the two switches 85 and 82, the

switch 82 is also operated to its off-normal position in response to the operation of the switch 85 to its oiI-normal position. Thus, the sp ngs 83 and 84 are closed when the switch 85is operated to its off-normal position. At the springs 83, the;

input circuit to the power supply apparatusis completed and, at the springs 84, the bridging path, traced above, is completed between ,the line conductors l6 and I1, whereby theoperated connector switch in the exchange 10 is caused to operate to interrupt the ringing operation, to complete the talking connection between the calling and called substations, and to impress talking battery voltage ,betweenthe conductors I6 and H of the line |2.. In response to the completion of the input circuit to the power supply apparatus, the various secondary windings of the transformer 55 are energized, whereby energizing current is delivered to the cathode heaters respectively embodied in the tubes 28, 41 and- 62, and anode voltage is developed between the positive and negative output conductors of the rectifier to supply anode and screen potentials to each of the two tubes 28 and 41. Thus, the

two amplifiers comprising, respectively, ,thetubes 28 and 41, are conditioned for operation. When the secondary winding 59 isenergized, energizing current is delivered to the signal lamp 60, thus,

anode current starts to flow in this circuit, the relay 12 operates to interrupt, at the armature 16 and its associated resting contact", the above-traced circuit for conducting incoming audio frequency currents to the receiver 19. Normally, this circuit is shunted across the nonadjustable "terminals of the potentiometer resistor 32. the shunting path including the receiver [9 is interruptedso that the audio voltage developed across the inductance element 38 causes a cur rent totraverse only the resistor 32.. The portion of this voltage developed across the adjustable portion of the resistor 32 is impressed between the input electrodes of the tube 28. is amplified in this tube, and the amplified audio ener' the auxiliary sound reproducing unit inactive; The relay 12, upon operating, not only substi--" When the relay 12 operates, however,

29 to the loud speaker 21 for reproduction. It

is noted that the reslstanceof the resistor 323s;

fchosen to have a value of the 'o'rder'of 100,000 ohms which is relatively high as comparedjto the highest impedance of the receiver 59," where by this resistor does not lay-pass ors'hunt -ifronrthe receiver l9 any substantial portion-oi the audio energy when the receiver is operatively in cluded'in the circuit and the signal *arnpliiieroi' tutes the auxiliary sound reproducing unit io'r the receiver 19 in the substation circuit, but also operatively includes the" second amplifier "com prising the vacuum tube 4! in the'channel' for transmitting audio frequencyenergy from ithe;

transmitter l8 to the line I29 More particularly, J

when t e y" operates, it interrupts, at tiie-- armature'15and its associated resting-contact, the above-traced closed link circuit including the condenser and connects, "at thearmature lfi andits associated working contact 13, the out' put electrodes of the tube 41' directly acres primary winding *43 connected inseries-with the source of anode voltage. As indicated above, the input electrodes of the tube 41 are permanently" coupled across the adjustable portion or the is energized, it is conditioned to ampliiythe audio voltage appearing across the indicated portion er the anti-side-tone impedance networlr33' t-o the line I 2. Itis pointed out that the resistance" of P the potentiometer resistor 51 is relatively high,

' tentiometer resistor 5i so that, when the tube. 11

being of the order of 100,000 ohmsk Also; when the vacuum tube 41 is inactive, the admittance between the input electrodes t8 and fifl thereof is-exceptionally low. Thus, only a small pertion of the audio frequency energy developed by? operation of the transmitter I8'is dissipated inf theresistor 5i and the tube '41. I

It will be noted that "when 'thexpower s pp yg apparatus is energized, the windingof-therel'ay 12 is initiallyenergized only by the currenttrav ersing the cathode leg of the output circuit'ior the tube 28. Thus, until the relay 12 operates,

the output circuit for the tube 41.15 held open at the. armature 15 and its associated working contact 19. This means that a relatively low' biasing voltage is initially impressed between the input electrodes of the tube '28 and, hence, the

anode current of this tube 28, which current traverses the winding of the relay", is rel atively large. When the relay," operates, the "1 output circuit for the tube'41 is completed in the mannerpointed out above, so that the current traversing the wlndingof the relay 12 is the tween the input electrodes of each of the tubes 28 and 41 is determined by the rriagnitudeof the sum'of the anode currentsrespectivelyflowing.

through the two tubes.

When the conversationis terminated and. the" connection is released, the switch 851s restored to its normal position to recomplete. atl'the' springs 85, the abovedlraced 10w impedance path, I comprising the condenser 8i for shunting the transmitter (8. Due to the above-describedme:

chanical interlock between the switches 82 and,

85, the switch 82 is also restored to normal response to the operation of the switch to its normal position. When the switch 82 is returned to normal, the springs 83 and 84 are opened respectively to interrupt the input circuit to the power supply apparatus and the bridging path between the conductors of the line l2. If the answering party fails to restore the switch 85 to normal, the energized condition of the lamp 60 is a visual reminder that the substation B is still operatively connected to the line [2. Obviously, when the circuit to the power supply apparatus is interrupted, current is no longer supplied to the cathode heaters of the tubes 28, 41 and 62, and the two signal amplifiers are rendered inactive. The resulting deenergization of the winding of the relay 12 causes this relay operatively to include the receiver IS in the substation circuit and effectively to exclude the auxiliary sound reproducing unit from the circuit. At the armature I5 and its associated 'working contact 19, the relay I2 disconnects the output electrodes of the tube 41 from across the primary winding 43 of the transformer 40 and, at the armature l5 and its associated resting contact 18, the relay 12 recompletes the above-traced link circuit including the condenser 80. Thus, the substation apparatus is restored to normal.

If the receiver I9 is utilized as the receiving means, this receiver is restored to its supporting member 22 at the conclusion of the conversation in the usual manner, whereby the hook or cradle switch 2| is restored to its normal position. When the switch 2| is restored to normal, the springs 23 are opened to interrupt the bridging path between the conductors of the line I! and the springs 24 are closed to connect the low impedance condenser 8| in shunt with the transmitter l8. It will be apparent from the foregoing description that the auxiliary sound reproducing unit and the receiver l8 may alternately be used as the receiving means during a single conversation by the simple expedient of operating the answer switch 85 from its normal to its oil-normal position and vice versa.

In order to initiate a call at the substation B, intended for one of the other substations, such, for example, as the substation A, the calling party may optionally use the regular substation transmitter and receiver or the auxiliary equipment comprising the auxiliary sound reproducing unit and the amplifier for amplifying audio frequency currents outgoing from the substation. If the calling party elects to use the receiver i9 as the receiving means, this receiver is removed from its supporting member 22 to cause the hook switch 2| to be operated to its ofi-normal position, whereby the above-described bridging path between the conductors of the line |2 is completed to complete a loop circuit extending to the automatic switching equipment provided in the exchange l0. At the springs 24, the abovetraced low impedance path shunting the transmitter l8 and comprising the condenser 8|, is interrupted. When the loop circuit, including the two conductors of the line I2, is completed to the exchange In, an associated or assigned non-numerical switch provided in the exchange is caused to operate in the usual manner to extend the calling loop circuit through to one of the non-numerical switches, which latter switch functions to return dial tone over the established 1001) to the calling party. It will be noted that the impulsing springs 25 of the impulsing device 20 are serially included in the loop circuit extending to the operated switch train. During operation of the impulsing device, the impulsing springs 25 are alternately opened and closed in the usual manner, thereby to transmit impulses by way of the line |2 to the seized automatic switching equipment in the exchange In. This equipment responds to the impulses in the usual manner to cause the connection to be forwarded by way thereof to the called line I. During each operation of the impulsing device 20 or, more particularly, during each digit dialed at the substation B, the shunt springs 26 are closed to complete an obvious path for excluding the substation apparatus irom the impulsing circuit. When the operation of the impulsing device 20 is ended, the shunt springs 28 are opened in the usual manner, thereby to condition the substation circuit for voice frequency current transmission and reception. In this connection, it will be apparent that, if the selected called line I I is testedand found busy, busy-tone current is returned over the calling loop circuit to be reproduced by the receiver l8. If, on the other hand, the called line is idle and ringing current is projected thereover to signal the called party, ringback-tone current is conducted over the calling line and through the resistor I9 to indicate that the called substation is being rung. When the calling subscriber answers, the desired conversational connection is completed in the usual manner. Following the completion of, the connection, the paths traversed by the signal currents incoming and outgoing from the substation B are identical with those traced above.

In the event the calling party elects to use the auxiliary substation equipment, namely, the auxiliary sound reproducing unit and the outgoing signal amplifier comprising the vacuum tube 41, the receiver I8 is not removed from its supporting member 22, but the calling key switch 82 is operated to its off-normal position to close the springs 83 and 84. At the springs 84, the abovetraced bridging path between the conductors of the line I2 is completed and, at the springs' 83, the input circuit to the power supply apparatus is completed. Thus, the two signal ampliflers are conditioned for operation, the relay i2 is caused to operate to exclude the receiver I9 from the substation circuit and operatively to include the auxiliary sound reproducing unit in the circuit, and a non-numerical switch provided in the exchange I0 is caused to operate to extend the calling loop circuit through to a selected one of the numerical or impulse responsive switches. As indicated above, the mechanical interlock between the two key operated switches 82 and 85 is such that the lastmentioned key is not operated to its off-normal position in response to the operation of the key 82'to its oil-normal position. Hence, the springs 86 are retained in engagement to hold completed the low impedance path shunting the transmitter i8. The purpose of this path is to prevent the substation apparatus from singing or oscillating locally during the period of unbalance of the anti-side tone impedance network 33. More particularly, when the call is initiated and until the call is answered at the called substation, the impedance provided by the series connected condenser 4| and resistor 42 is not balanced by the equivalent impedance, measured at the substation terminals between the line conductors l6 and ll, of the impedances associated with the line i2 and external to the substation B. Thus, the anti-side-tone impedance network is unbalanced and electrical coupling is inadvertently provided between the output circuit reproducing device, a. circuit including a perof the signal amplifier comprising the tube 41 and the input circuit of the signaling amplifier comprising the tube 28. Also, acoustical coupling exists between the loud speaker 21 and the transmitter l8. By virtue of these two couplings, a closed, local oscillatory circuit comprising the two signal amplifiers, theloud speaker 21 and the transmitter I8, is created during the interval whenthe anti-side-tone network is unbalanced. Through the provision of the low impedance path shunting the transmitter I8 and comprising the condenser 8|, however, this local circuit is effectively interrupted because substantially no input energy can-be delivered to the-signal amplifier comprising the vacuum tube 41. Hence,

the system is prevented from oscillating, and singing or howling is minimized.

Following the operation of the switch 82 to its off-normal position, the connection is routed by way of the automatic switching equipment provided in the exchange l0 to the called line H in response to operation of the impulsing device 20 in the exact manner described above. When the call is answered at the called substation A, the desired conversational circuit is completed, and the anti-side-tone impedance network 33 is balanced, so that the electrical cou.- pling as described above and-included in the local substation circuit is substantially eliminated.

Obviously, when the call is answered, incoming audio frequency currents are reproduced by the auxiliary sound reproducing unit even though the transmitter I8 is effectively shunted by the low impedance path described above. When the call is answered, the calling party may condition the substation circuit for the transmission of audio frequency currents therefrom by operating the answer switch 85 to its off-normal position,

thereby to interrupt, at the springs 86, the low impedance path shunting the transmitter Hi.

In order to release the connection, as established in the above-described' manner, after the conversation is terminated, it is only necessary for the calling party to return the receiver I9 toits supporting member 22 or to operate the answer key 85 to its normal position, depending ment of the invention, it will be understood that various modifications may be made therein, and it is,,inten ded to cover inthe appended claims all such modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. A telephone substation adapted to be connected to a telephone line comprising a pair of conductors arranged tohave a source of direct current connected thereto and comprising, in combination, a transmitter, a sound reproducing device and an anti-side-tone impedance network arranged to be interconnected to form a substation circuit, said anti-side-tone impedance network comprising a plurality of impedance elements and being operative toprevent audio frequency energy developed by operation ofsaid transmitter from being delivered to said sound tion of said impedance elements and the conaudio frequency energy developed by operation of said transmitter from being delivered to said line oversaid circuit,'an auxiliary sound reproducing unit comprising an amplifier, said amplifie'r comprising a vacuum tube having input electrodes, a second amplifier including avacuum tube having input electrodes, ajre'lay operative to substitute said auxiliary sound reproducing unit for said sound reproducing device in saidsubstation circuit, said relay also being operative to include said second amplifier in said substation circuit, means including said second work for delivering audio frequency energy de-' veloped byoperation of said transmitter to said line,.and means including said relay for impress' amplifier and said anti-side tone impedance neting abiasing voltage between the input electrcdes I of atleast one of said tubes.

2. A telephone substation'adapted to-lbeconnected to a telephone line comprising a pair of conductors arranged to have a source of'direct current connected thereto and comprising, in

combination, fa transmitter, a sound reproduc ing device and an anti-side-tone impedance nctwork arranged to be interconnected toform a substation circuit, saidanti-side-tone impedance network comprising a plurality of impedance elements and being operative to prevent audio fre' quency energy developed by operation of said transmitter from being delivered to said sound reproducing device, a circuit including a portion of said impedance elements and the conductors of said line for conducting energizing current from said source to said transmitter, said circuit ,also including a network for preventing audio frequency energy developed by operationof said transmitter from being delivered toisaid line over said circuit, an auxiliary sound reproducing unit 1 comprising an amplifier, said amplifier comprising a vacuum tube having input electrodes, a

second amplifier including a vacuum tube having I a input electrodes, a relay operative to substitute said auxiliary sound reproducing unit for said reproducing device in said substation circuit, said relay also being operative to include said second amplifier in said substation circuit, means in-'- cluding said second amplifier and said anti-sidetone impedance network for delivering audio frequency energy developed by operation of said transmitter to said line, said relay having a winding, and circuits individual to said tubes and commonly including said winding for impressing a biasing voltage between the input electrodes of each of said tubes.

3. A telephone substation adapted to be con nected to a telephone line comprising a pair of conductors arranged to have a source of direct current connected thereto and comprising, in

combination, a transmitter, a sound reproducing device and an anti-side-tone impedance network arranged to be interconnected to form a substation circuit, said anti-side-tone impedance, network comprising a plurality of impedance elements and being operative to prevent audio frequency energy developed by operation of said transmitter from being delivered to said sound reproducing device, a circuit including a portion of said impedance elements and the conductors of said line for conducting energizing current from said source to said transmitter, said circuit also including a network for preventing audio frequency energy developed by operation of said transmitter from being delivered to said line over said circuit, an auxiliary sound reproducing unit comprising an amplifier, said amplifier comprising a vacuum tube having input electrodes, a

second amplifier including a vacuum tube having input electrodes, a relay operative to substitute said auxiliary sound reproducing unit for said sound reproducing device in said substation circuit, said relay also being operative to include said second amplifier in said substation circuit, means including said second amplifier and said anti-side-tone impedance network for delivering audio frequency energy developed by operation of said transmitter to said line, power supply apparatus including an input circuit, a switch operable to complete said input circuit, said relay including a winding connected and arranged to be energized in response to the completion of said input circuit, and circuits individual to said tubes and commonly including said winding for impressing a biasing voltage between the input electrodes of each of said tubes.

4. A telephone substation adapted to be connected to a telephone line comprising a pair of conductors arranged to have a source of direct current connected thereto and comprising, in combination, a transmitter, a sound reproducing device and an anti-side-tone impedance network arranged to be interconnected to form a substation circuit, said anti-side-tone impedance network comr -sing four coupled inductance elements connected to form a bridge including four arms and two conjugate terminals, a transformer incudirlg a primary winding normally coupled to said transmitter and ,a secondary winding coupled to said conjugate terminals, whereby currents traversing said secondary winding diide and traverse two of the arms of said bridge in opposite directions, said anti-side-tone impedance network including another inductance element inductively coupled to said two arms of said bridge and normally coupled to said sound reproducing device, whereby audio frequency energy developed by operation of said transmitter is not delivered to said sound reproducing device, a circuit including two arms of said bridge and the conductors of said line for conducting energizing current from said source to said transmitter, said last-named circuit also including a network for preventing audio frequency energy developed by operation of said transmitter from being delivered to said line over said last-named circuit, an auxiliary sound reproducing unit including an amplifier, said amplifier comprising a vacuum tube having input electrodes, a second amplifier having output terminals andincluding a vacuum tube having input electrodes coupled to said transmitter, a relay operative to substitute said auxiliary sound reproducing unit for said sound reproducing device in said substation circuit, said relay also being operative to couple the output terminals of said second amplifier to the primary winding of said transformer, whereby said second amplifier and said anti-side-tone impedance network are conditioned to deliver audio frequency energy developed by operation of said transmitter to said line, and means including said relay for impressing a biasing voltage between the input electrodes of at least one of said tubes.

5. A telephone substation adapted to be connected to a telephone line comprising a pair of conductors arranged to have a source of direct current connected thereto and comprising, in combination, a transmitter, a sound reproducing device and an anti-side-tone impedance network arranged to be interconnected to form a substation circuit, said anti-side-tone impedance network comprising four coupled inductance elements connected to form a bridge including four arms and two conjugate terminals, a transformer including a primary winding normally coupled to said transmitter and a secondary winding coupled to said conjugate terminals, whereby currents traversing said secondary winding divide and traverse-two of the arms of said bridge in opposite directions, said anti-side-tone impedance network including another inductance element inductively coupled to said two arms of said bridge and normally coupled to said sound reproducing device, whereby audio frequency energy developed by operation of said transmitter is not delivered to said sound reproducing device, a circuit including two arms of said bridge and the conductors of said line for conducting energizing current from said source to said transmitter, said last-named circuit also including a network for preventing audio frequency energy developed by operation of said transmitter from being delivered to said line over said last-named circuit, an auxiliary sound reproducing unit including an amplifier, said amplifier comprising a vacuum tube having input electrodes, a second amplifier having output terminals and including a vacuum tube having input electrodes coupled to said transmitter, a relay operative to substitute said auxiliary sound reproducing unit for said sound reproducing device in said substation circuit,'said relay also being operative to couple the output terminals of said second amplifier to the primary winding of said transformer, whereby said second amplifier and said anti-side-tone impedance network are conditioned to deliver audio frequency energy developed by operation of said transmitter to said line, said relay having a winding, and circuits individual to said tubes and commonly including said winding for impressing a biasing voltage between the input electrodes of each of said tubes.

6. A telephone substation adapted to be connected to a telephone line comprising a pair of conductors arranged to have a source of direct current connected thereto and comprising, in combination, a transmitter, a sound reproducing device and an anti-side-tone impedance network arranged to be interconnected to form a substation circuit, said anti-side-tone impedance network comprising four coupled inductance elements connected to form a bridge including four arms and two conjugate terminals, a transformer including a primary winding normally coupled to said transmitter and a secondary winding coupled to said conjugate terminals, whereby currents traversing said secondary winding divide and traverse two of the arms of said bridge in opposite directions, said anti-side-tone impedance net- Work including another inductance element inductively coupled to said two arms of said bridge and normally coupled to said sound reproducing device, whereby audio frequency energy developed by operation of said transmitter is not delivered to said sound reproducing device, a circuit including two arms of said bridge and the conductors of said line for conducting energizing current from said source to said transmitter,

said last-named circuit also including a network for preventing audio frequency energy developed by operation of said transmitter lrom being delivered to said line over said last-named circuit, an auxiliary sound reproducing unit including an amplifier, said amplifier comprising a vacuum tube having input electrodes, a second amplifier having output terminals and including a vacuum tube having input electrodes coupled to said transmitter, a relay operative to substitute said auxiliary sound reproducing unit for said sound reproducing device in said substation circuit, said relay also being operative to couple the output terminals of said second amplifier to the primary winding of said transformer, whereby said second amplifier and said anti-side-tone impedance network are conditioned to deliver audio frequency energy developed by operation of said transmitter to said line, power supply apparatus including an input circuit, a switch operable to complete said input circuit, said relay including a Winding connected and arranged to be energized in response to the completion of said input circuit, and circuits individual to said tubes and commonly including said winding for impressing a biasing voltage between the input electrodes of each of said tubes.

7. A telephone substation adapted to be connected to a telephone line comprising a pair ofconductors arranged to have a source of direct current connected thereto and comprising, in combination, a transmitter, a sound reproducing device and an anti-side-tone impedance network arranged to be interconnected to form a substation circuit, said anti-side-tone impedance network comprising a plurality of impedance elements and being operative to prevent audio frequency energy developed by operation of said transmitter from being delivered to said sound reproducing device, a circuit including a portion of said impedance elements and the conductors of said line for conducting energizing current from said source to said transmitter, said circuit also including a network for preventing audio frequency energy developed-by operation of said transmitter from being delivered to said line over said circuit, additional means including said antiside-tone impedance network for delivering audio frequency energy developed by operation of said transmitter to said line, an auxiliary sound reproducing unit comprising an amplifier, said amplifier comprising avacuum tube having input electrodes, a relay operative to substitute said auxiliary sound reproducing unit for said sound reproducing device in said substation circuit, and means including said relay for impressing a biasing voltage between the input electrodes of said vacuum tube. A

8. A telephone substation adapted to be connected to a telephone line comprising a pair of conductors arranged to have a source of direct current connected thereto and comprising, in combination, a transmitter, a sound reproducing device and an anti-side-tone impedance network arranged to be interconnected to form a substation circuit, said anti-side-tone impedance network comprising a plurality of impedance elements and being operative to prevent audio frequency energy developed by operation ofsaid transmitter from being delivered to said sound reproducing device, a circuit including a portion of said impedance elements and the conductors of said line for conducting energizing current from said source to said transmitter, said circuit also including a network for preventing audio frequency energy developed by operation of said transmitter from being delivered to said line over said circuit, additional means including said anti side-tone impedance network for delivering audio frequency energy developed by operation of said transmitter to said line, an auxiliary sound reproducing unit comprising an amplifier, said am-.

,plifier comprising a vacuum tube having input electrodes, a relay operative to substitutesaid auxiliary sound reproducing unit for said sound reproducingdevice in said substation circuit, said relay having a winding, and a circuit including said winding for impressing a biasing voltage between the input electrodes of said vacuum tube.

9. A telephone substation adapted to beconnected to a telephone line comprising a pair of conductors arranged to have'a source of direct current connected thereto and comprising, in combination, a transmitter. a sound reproducing device and an anti-side-tone impedance network arranged to be interconnected to form a substation circuit, said anti-side-tone impedance net" work comprising a plurality of impedance elements and being operative to prevent audio frequency energy developed by operation of said transmitter from being delivered to said sound reproducing device, a circuit including a portion of said impedance elements and the conductors of said line for conducting energizing current from said source to said transmitter, said circuit also including a network for preventing audio frequency energydeveloped by operation of said transmitter from being delivered to said line over said circuit, additional means including said antiside-tone impedance network for delivering audio frequency energy developed'by operation of said transmitter to said line, an auxiliary sound reproducing unit comprising an amplifier, said amplifier comprising a vacuum tube having input electrodes, a relay operative to substitute said auxiliary sound reproducing unit for said sound reproducing device in said substation circuit, power supply apparatus including an input circuit, a switch operable to complete said input circuit, said relay including a winding adapted to be energized in response to the completion of said input circuit, and a circuit including said winding for impressing a biasing voltage between the input electrodes of said vacuum tube.

10. A telephone substation adapted to be connected to a telephone line comprising a pair of conductors arranged to have a source of direct current connected thereto and comprising, in combination, a transmitter, a sound reproducing device and an anti-side-tone impedance network arranged to be interconnected to form a substation circuit, said anti-side-tone impedance network comprising four coupled inductance elements connected toiorm a bridge including four arms and two conjugateterminals, a transformer including a' primary winding normally coupled to said transmitter and a secondary winding coupled to said conjugate terminals, whereby currents traversing said secondary winding divide and traverse two arms of said bridge in opposite directions, said anti-side-tone impedance network including another inductance element inductively coupled to said two arms and normally coupled to said sound reproducing device, whereby audio frequency energy developed by operation of said transmitter is not delivered to said sound reproducing device, a circuit including two arms (if said bridge and the conductors of said line for conducting energizing current from said source to said transmitter, said last-named circuit also including anetwork for preventing audio frequency energy developed by operation of said transmitter from being delivered by way of said last-named circuit to said line, additional means including said anti-side-tone impedance network for delivering audio frequency energy developed by operation of said transmitter to said line, an auxiliary sound reproducing unit comprising an amplifier, said amplifier comprising a vacuum tube having input electrodes, a relay operative to substitute said auxiliary sound reproducing unit for said sound reproducing device in said substation circuit, and means including said relay for impressing a biasin voltage between the input electrodes of said vacuum tube.

11. A telephone substation adapted to be connected to a telephone line comprising a pair of conductors arranged to have a source of direct current connected thereto and comprising, in combination, a transmitter, a sound reproducing device and an anti-side-tone impedance network arranged to be interconnected to form a substation circuit, said anti-side-tone impedance network comprising four coupled inductance elements connected to form a bridge including four arms and two conjugate terminals, a transformer including a primary winding normally coupled to said transmitter and a secondary winding coupled to said conjugate terminals, whereby currents traversing said secondary winding divide and traverse two arms of said bridge in opposite directions, said anti-side-tone impedance network including another inductance element inductively coupled to said two arms and normally coupled to said sound reproducing device, whereby audio frequency energy developed by operation of said transmitter is not delivered to said sound reproducing device, a circuit including two arms of said bridge and the conductors of said line for conducting energizing current. from said source to said transmitter, said last-named circuit also including a network for preventing audio frequency energy developed by operation of said transmitter from being delivered by way of said last-named circuit to said line, additional means including said anti-side-tone impedance network for delivering audio frequency energy developed by operation of said transmitter to said line, an auxiliary sound reproducing unit comprising an amplifier, said amplifier comprising a vacuum tube having input electrodes, a relay operative to substitute said auxiliary sound reproducing unit for said sound reproducing device in said substation circuit, said relay having a winding, and a circuit including said winding for impressing a biasing voltage between the input electrodes of said vacuum tube.

12. A telephone substation adapted to be connected to'a telephone line comprising a pair of conductors arranged to have a source of direct current connected thereto and comprising, in combination, a transmitter, a sound reproducing device and an anti-side-tone impedance network arranged to be interconnected to form a substation circuit, said anti-side-tone impedance network comprising four coupled inductance elements connected to form a bridge including four arms and two conjugate terminals, a transformer including a primary winding normally coupled to said transmitter and a secondary winding coupled to said conjugate terminals, whereby currents traversing said secondary winding divide and traverse two arms of said bridge in opposite directions, said anti-side-tone impedance network including another inductance element inductively coupled to said two arms and normally coupled to said sound reproducing device,

whereby audio frequency energy developed by operation of said transmitter is not delivered to said sound reproducing device, a circuit including two arms of said bridge and the conductors of said line for conducting energizing current from said source to said transmitter, said last-named circuit also including a network for preventing audio frequency energy developed by operation of said transmitter from being delivered by way of said last-named circuit to said line, additional means including said anti-side-tone impedance network for delivering audio frequency energy developed by operation of said transmitter to said line, an auxiliary sound reproducing unit comprising an amplifier, said amplifier comprising avacuumtube having input electrodes, a relay operative to substitute said auxiliary sound reproducing unit for said sound reproducing device in said substation circuit, power supply apparatus including an input circuit, a switch operable to complete said input circuit, said relay including a winding adapted to be energized in response to the completion of said input circuit, and a circuit including said winding for impressing a biasing voltage between the input electrodes of said vacuum tube.

13. A telephone substation adapted to be connected to a telephone line comprising a pair of conductors arranged to have a source of direct current connected thereto and comprising, in combination, a. transmitter, a circuit including the conductors of said line for conducting energizing current from said source to said transmitter, said last-named circuit comprising a network for preventing audio frequency energy developed by operation of said transmitter from being delivered by way of said last-named circuit to said line, and additional means for delivering audio frequency energy de eloped by operation of said transmitter to said line.

14. A telephone substation adapted to be connected to a telephone line comprising a pair of conductors arranged to have a source of direct current connected thereto and comprising. in combination, a transmitter, a circuit including the conductors of said line for conducting energizing current from said source to said transmitter, said last-named circuit comprising a low impedance alternating current by-passing and blocking network for preventing audio frequency energy developed by operation of said transmitter from being delivered by way of said lastnamed circuit to said line, and additional means for delivering audio frequency energy developed by operation of said transmitter to said line.

15. A telephone substation adapted to be connected to a telephone line comprising a pair of conductors arranged to have a source of direct current connected thereto and comprising, in combination, a transmitter, a sound reproducing device and an anti-side-tone impedance network arranged to be interconnected to form a substation circuit, said anti-side-tone impedance network comprising a plurality of impedance elements and being operative to prevent audio frequency energy developed by operation of said transmitter from being delivered to said sound reproducing device, a circuit including a portion of said impedance elements and the conductors of said line for conducting energizing current from said source to said transmitter, said lastnamed circuit comprising a network for preventing audio frequency energy developed by operation of said transmitter from being delivered to said line through said last-named circuit, and

additional means including said anti-side-tone impedance network for delivering audio irequencyenergy developed by operation of said transmitter tosaid line.

16. A telephone substation adapted to be connected: to a telephone line comprising a pair of conductors, arranged to have a source of direct. current connected thereto and comprising in combinationa transmitter, a soundreproducing,

device and an anti-side-tone impedance :network :arranged to be interconnected'todor'm a substation circuit, said anti-side-tone impedance I network comprising'four coupled inductance elements connected to form a bridge including four arms and twoconjugate terminals, a transformer normally coupled to said sound reproducing de- 7 vice, whereby audio frequencyenergy developed by operation of said transmitter is not delivered to said sound reproducing device, a circuit including two arms of said bridge and the conductors of said line for conducting energizing current from said source to said transmitter,said

last-named circuit also including a network forpreventing audio frequency energy developed by operation of said transmitter from being delivered by way of. said last-named circuit to said line, and additional means including said anti-sidetone impedance network for delivering audio frequency energy to said line,

17.,A telephone substation adapted to be con nected to a telephone line and comprising, in combination, a transmitter and receiver arranged to be interconnected. to form a substation circuit, an auxiliary sound reproducing unit comprising an amplifier, said amplifier including a vacuum tube having input electrodes, a relay operative to substitute said auxiliary sound reproducing unit for said receiver in said substation circuit, said relay including a winding, and a circuit including said winding for impressing a direct biasing voltage between the input electrodes of said. tube when said winding is energized.

18. A telephone substation adapted to be connected to a telephone line and comprising, in combination, a transmitter and receiver arranged to be interconnected to form a substation circuit, an auxiliary so'undreproducing unit comprising an amplifier, said amplifier including a vacuum tube having input electrodes, a relay operative to substitute said auxiliary sound reproducing unit for 'said receiver in said substation circuit, said relay including a winding, power supply apparatus including an input circuit, a switch operable to complete said input circuit, said winding being connected and arranged to be energized in response to the completion of said input circuit, and a circuit including said winding for impressing a direct biasing voltage between the input electrodes of said tube when said winding is energized.

19. A telephone substation adapted to be con nected to a telephone line, and comprising, in combination, a transmitter and receiver arranged to be interconnected toform a substation circuit. an auxiliary sound reproducing unit comprising an amplifier, said amplifier including a. vacuum tube having input electrodes, a second amplifier including a vacuum tube having input electrodes, a relay operative to substitute. said auxiliary sound reproducing unit for said receiver in said substation circuit, said relay also being operative to include said second amplifier in said substation circuit, means including said second amplifier for delivering audio frequency energy developed through operation of said transmltter'to' said. line, and means including said relay for impressing a biasing voltage between the input electrodes of each of said tubes.

2.0. A telephone substation adapted to be connected to a telephone line and comprising, in

combination, a transmitter and receiver arranged i I "to be interconnected to form a substation circuit, an auxiliary sound reproducing unit comprising an amplifier, said amplifier including a vacuum t-ubehaving input electrodes, a second amplifier including a vacuum tube having input electrodes, a relay operative to substitute said auxiliary sound reproducing unit for said receiver in said substation circuit, said relayalso being operative 'to include said sec nd amplifier in said substation circuit, means including said second amplifier for delivering audio frequency energy developed through operation of said transmitter to said, line, said relay including a windin and d individual to said tubes and commonly including said winding for impressing a biasing voltage between the input electrodes ofeach of said tubes.

21. A telephone substation adapted to be connected to a telephone lineand comprising, in

combinations transmitter and receiver arranged to be interconnected to form a substation circuit,

an auxiliary sound reproducing unit comprising i an amplifier, said amplifier including a vacuum tube having input electrodes, 9, second amplifier including a vacuum tube having input electrodes, a relay operative to substitute said auxiliary sound reproducing unit for said receiver in said substation circuit, said relay also being operative to include said second amplifier in said substation circuit, means including said second amplifier for delivering audio frequency energy developed through operation of said transmitter to said line, said relay including a winding, power supply apparatus including an input circuit, a switch operable to complete said circuit, said winding being connected and arranged to be energized in response to the completion of said input circuit, and circuits individual to said tubes and commonly including said winding for impressing a biasing voltage between the input electrodes of each of said tubes.

22. A telephone substation adapted to be connected to a telephone line and comprising, in combination, a transmitter and receiver arranged to be interconnected to form a substation circuit, an amplifier including a vacuum tube having input electrodes, said transmitter normally being connected to transmit audio frequency energy to said line over a path from which said amplifier isexclude-d, a relay operative to include said amplifier in said audio frequency energy transmisput electrodes, said transmitter normally being connected to transmit audio frequency energy to said line over a path from which said amplifier is excluded, a relay operative to include said amplifier in said audio frequency energy transmission path, power supply apparatus including an input circuit, a switch operable to complete said circuit, said winding being connected and arranged to be energized in response to the completion of said input circuit, and a circuit including said winding for impressing a direct biasing voltage between the input electrodes of said tube when said winding is energized.

24. A telephone substation adapted to be connected to a telephone line and comprising, in combination, a transmitter and receiver arranged to be interconnected to form a substation circuit, a path for directly transmitting audio frequency energy from said transmitter to said line, an amplifier having input terminals coupled to said transmitter and output terminals adapted to be coupled to said line, power supply apparatus operative to deliver energizing current to said amplifier, thereby to condition said amplifier for operation, an input circuit for said power supply apparatus, a switch for completing said input circuit, and means responsive to the operation of said switch for simultaneously interrupting said path and coupling said output terminals to said line. .c

25. A telephone substation adapted to be connected to a telephone line and comprising, in combination, a transmitter and receiver arranged to be interconnected to form a substation circuit, a path for directly transmitting audio frequency energy from said transmitter to said line, an

amplifier having input terminals coupled to said transmitter and output terminals adapted to be coupled to said line, power supply apparatus operative to deliver energizing current to said amplifier, thereby to condition said amplifier for operation, an input circuit for said power supply apparatus, a switch for completing said input circuit, and a relay operative simultaneously to interrupt said path and to couple said output terminals to said line, said relay including a winding connected and arranged to be energized in response to the completion of said input circuit.

26. A telephone substation adapted to be coiinected to a telephone line and comprising, in combination, a transmitter and a receiver arranged to be interconnected to form a substation circuit, an auxiliary sound reproducing unit, a switch operable from a normal position to an off-normal position, means responsive to the operation of said switch to said oii-normal position for substituting said auxiliary sound reproducing unit for said receiver in said substation circuit, a low impedance path normally shunting said transmitter, a second switch operable from a normal position to an off-normal position, means responsive to the operation of said second switch to said off-normal position for interrupting said low impedance path, and means responsive to the operation of said first-named switch from its off-normal position to its normal position for operating said second switch from its off-normal position to its normal position.

'27. A telephone substation adapted to be conneceted to a telephone line and comprising, in combination, a transmitter, a receiver and an anti-side-tone impedance network arranged to be interconnected to form a substation circuit, said anti-side-tone impedance network being operative to prevent audio frequency energy developed by operation of said transmitter from being delivered to said sound reproducing device, an auxiliary sound reproducing ,unit, a switch operable from a normal position to an onnormal position, means responsive to the operation of said switch to said off-normal position for substituting said auxiliary sound reproducing unit for said receiver in said substation circuit and for creating a calling condition on said line, a low impedance path normally shunting said transmitter, a second switch operable from a normal position to an off-normal position, means responsive to the operation of said second switch to its oiT-normal position for interrupting said low impedance path, and means responsive to the operation of said first-named switch from its off-normal position to its normal position for operating said second switch from its off-normal position to its normal position.

ROSWELL HARRY HERRICK. TURNER WRIGHT GILMAN. 

